The diagnostic value of the fine structure lines have been quantified
in terms of lines detectable above the continuum, for the spectrometers
ISO-LWS and ISO-SWS, for realistic instrumental resolution. The number of
detectable lines depends on
the type of exciting star, the radial density profile and the
absolute values for the densities of the gas and the dust,
with relative abundance of elements being the same for all models.
In general, the number of detectable lines (D > -1.00) reduces as one
goes from O4 to O7 to B0.5, as the luminosity and
effective temperature of the embedded star reduces.
Also for the r0 case, the detectable lines are more
numerous than r-1 case. Similarly r-1 case
has more lines than the r-2 case in general.

In addition to the absolute luminosities of these lines,
it is important to know the line to continuum contrast,
the parameter D, which
identifies lines that are more easily detectable.
The values of D are also listed in
Table 6, along with the line
luminosities, for the selected models.

Figure 8. The emergent spectra
from a spherical interstellar cloud with an embedded ZAMS star of O4 type.
The wavelengths for a few prominent fine structure lines
are marked near the abscissae. (a) The spectra for the
case of uniform radial density distribution (r0),
and demonstrate the role of the optical depth. The three spectra
correspond to the values of
100 to be
(i) 0.0045 (bottom), (ii) 0.075 (middle) and (iii) 0.34 (top).
The curves (ii) & (iii) have been shifted by +3.0 (dex) & +6.0
(dex) respectively for clarity. (b) The spectra correspond to the case of
100 = 0.0045,
and show the effect of different radial density distributions.
The three spectra refer to the cases of (i) r0 (bottom),
(ii) r-1 (middle) & (iii) r-2 (top).
The curves (ii) & (iii) have been shifted by +3.0 (dex) & +6.0
(dex) respectively for clarity.

We note that for O4 star and r0 density distribution
(Table 6), the [O III] (51.8
µm) line happens
to be the most luminous. However, from the viewpoint of the line to
continuum ratio, for the same family of models,
the [O III] (88.4 µm) line is found to have the best
detectability. This suggests that while the absolute
luminosity of the lines are helpful for
understanding the different physical parameters, the relative
contrast of the lines with respect to the local continuum bear
useful information about the actual appearance of the
spectrum and hence the detectability of a particular line.
The latter is extremely useful from the observation point of view.
Based on the results presented here
(Table 6 &
Figs. 8 - 10), many
quantitative inferences can be drawn, like the following :
(i) exact luminosity as well as detectability (for the ISO
spectrometers) sequence of spectral lines; (ii) role of a
particular line in cooling of the interstellar medium (ISM) near
H II regions; (iii) relative roles of different elements in
the energetics of the ISM; (iv) expected line ratios for the same
species (being abundance independent) under different physical
conditions in / near H II regions, as a function of optical
depth & density distribution; (v) the role of the continuum
from the dust, in highlighting or suppressing expected
detection of various fine structure lines; etc.

Figure 9. Same as in Fig. 8, but
for the case of a ZAMS star of O7 type embedded in the cloud.

From the above it is very clear that the details of
optical depth, the exciting stellar type (luminosity and effective
temperature) as well as the dust density distribution are well
imprinted on the emergent line spectrum of the compact
H II region. Hence, the present study has quantitatively
demonstrated the diagnostic value of several fine
structure lines of abundant elements in the interstellar medium. The same
can be used extensively in studies dealing with ISO
spectrometer in near future.

Figure 10. Same as in Fig. 8, but
for the case of a ZAMS star of B0.5 type embedded in the cloud.

Within the published ISO-SWS spectra, though with limited wavelength
coverage of compact H II regions
(Roelfsema et
al. 1996) we notice that one of their sources viz.,
IRAS 21190+5140,
approximately matches our prediction for a O7 star embedded
in a uniform density of total radial
100µm = 0.0045.
Not only the two line luminosities at 8.99 µm [Ar III]
and 10.5 µm [SIV], but also the nearby continuum are
roughly matching the predictions. This is only to indicate
the usefulness of the present results. Obviously a
finer tuning of the parameters near these values would be needed before
firm conclusions can be drawn about this source.